Portable electronic devices, such as mobile phones, personal digital assistants (PDAs) and digital cameras integrate more and more functionality in order to provide a broad variety of features. In particular, there are highly integrated cameras having integrated flash lights, and there are mobile phones that include digital, high resolution cameras with integrated flash lights. The light source used for the flash is capable of appropriate illumination within a target range greater than 150 Lux within one meter. Meanwhile, semiconductor technology has advanced to an extent that LEDs can be used as light sources for camera flash operation. However, the LEDs used for this purpose are typically driven with a peak current. The battery-driven hand-held mobile devices have to provide these high output currents for several hundred milliseconds. Dependent on the charging state of the battery, the battery's output impedance and other battery specific parameters, the high peak current loading can result in a battery voltage drop that can cause system failures.
In FIGS. 1 and 2 are graphs depicting battery voltage as a function of time for four different (V1 through V4) operating conditions of a portable device that generates a flash in a light-emitting semiconductor: a new battery at a temperature of 25° C.; a new battery at a temperature of −10° C.; a battery after 1000 charge/discharge cycles at −10° C.; and a battery after 1000 charge/discharge cycles at 25° C. FIG. 2 is an expanded view of the graph in FIG. 1 from 0-1.8 ms. The voltage drop can be divided into two phases, a first phase (within about 100 μs from the beginning of the high current phase dependant on the age and the temperature of the battery) and a second phase (from the end of the first phase until the end of the high current phase) where the voltage drop has exponential behavior. As the light-emitting semiconductor generates a flash, at t=0 ms, the battery voltage drops suddenly. In the newer batteries that are operating at warmer temperatures, the voltage does not drop any further than the initial voltage drop; however, the voltage level of the older batteries operating under colder conditions continues to drop throughout the duration of the flash, which can last from approximately 120 ms to 500 ms. The battery operating at −10° C. after 1000 cycles of use has the largest voltage drop, of around 0.6 V, whereas the new battery operating at 25° C. has the smallest voltage drop, of around 0.2 V. When the light-emitting semiconductor stops generating the flash, the voltage level of the battery increases again back to its original level before the start of the flash. In the newer batteries operating at higher temperatures, it can be seen that this happens almost instantaneously, whereas in the older batteries operating at lower temperatures (most particularly for the battery operating at −10° C. after 1000 cycles of use) it can take around 400 ms before the battery voltage is back to its original level.
Some examples of conventional devices are U.S. Patent Pre-Grant Pub. No. 2003/0076051, German Patent No. 10333418, and PCT Appl. No. WO2006/081613.